Purification of phosphoric acid



United States Patent Ofi ice 3,298,782 Patented Jan. 17, 1967 3,298,782PURIFICATION OF PHOSPHORIC ACID Jacques Olivier Archambault, Montreal,Quebec, Canada,

assignor to Canadian Industries Limited, MontreaLQuebee, Canada, acorporation of Canada No Drawing. Filed May 16, 1963, Ser. No. 281,065Claims priority, application Canada, May 24, 1962,

5 Claims. (Cl. 23-165) This invention relates to the extraction ofphosphoric acid from its aqueous solutions and, more particularly, tothe purification of wet-process phosphoric acid by a solvent extractionmethod. a

Wet-process phosphoric acid is'a'well-kn'own article of commerce and isproduced by the digestion of phosphate rock with sulphuric acid. It isnot normally possible to use'sulphuric acid of a strength greater thanthat which will. yield a phosphoric acid containing about 30% by weightof P since with more concentrated acid the precipitated calcium sulphatebecomes more diflicult to filter. The impurities present in theresulting wet-process phosphoric acid depend upon the source of thephosphate rock used, but generally include sulphates, fluorinederivatives, silicates and iron and aluminum salts.

It is known that phosphoric acid can be extracted from its aqueoussolutions by contact with an immiscible alcohol. It has now beendiscovered that this extraction can be made very much more efiicient,and thus commercially attractive, by the addition to the alcohol of aminor amount of a water-insoluble amine.

It is thus an object of this invention to provide an improved processfor extracting phosphoric acid from aqueous solutions thereof. Anotherobject is to" provide a process forthe purification of impure phosphoricacid solutions, particularly wet-process phosphoric acid. A furtherobject is to prepare phosphates from wet-process acid without anexpensive acid concentration step. Additional objects will appearhereinafter.

'The process of this invention broadly comprises contacting an aqueoussolution of phosphoric acid with a mixture of a minor amount of awater-insoluble amine and a major amount of an aliphatic alcohol havingfrom 4 to 12 carbon atoms.

More particularly, the process of this invention comprises contacting animpure aqueous phosphoric acid with a mixture of a minor amount of awater-soluble amine and a major amount of an aliphatic alcohol havingfrom 4 to 12 carbon atoms per molecule, separating the alcohol-aminephase With the major portion of the phosphoric acid solub-ilizedtherein, and recovering purified phosphoric acid from said alcohol-aminephase.

The alcohols used in the process are monohydric aliphatic alcohols, and,as stated hereinbefore, have from 4 to 12 carbon atoms per molecule.They may have straight or branched hydrocarbon chains and may beprimary, secondary or tertiary alcohols. Suitable alcoholsinclude-n-butanol, secondary butyl alcohol, amyl alcohol, n-hexanol,LI-dimethyl hexanol, nonanol, decanol and dodecanol.

The amines suitable for the process are of sufficiently carbon atoms. Inthis manner the loss of amine to the aqueous layer is minimized.Suitable amines thus include octadecylamine, methyl dioctylamine,dinonylamine, di-' lauryl butylamine, trilaurylamine and trinonylamine,and they may be used in any amount from an effective trace up to anequal weight with the alcohol. I It is preferred, however, to use theamine in an amount of from,5% to 20% by weight of the alcohol.

The contacting of the phosphoric acid solution with the immisciblealcohol-amine solution may be effected batchwise by stirring the twomaterials together, in which case the alcohol-amine phase can beseparated by decantation or in a separating funnel. Preferably, however,the con- 1 methods. If phosphoric acid is the required end-product,

then the organic mixture may be treated by back extraction with water.In this manner a purified phosphoric acid is obtained which is ofapproximately the same concentration in Water as the impure acidinitially used in the process. If phosphates are the desiredend-product;

then the organic mixture may be treated directly with the required base,for example ammonia, caustic soda, caustic potash, lime or magnesiumhydroxide, or with a salt whose anion is more easily removed fromsolution than is phosphate,such as a carbonate or a bicarbonate. Thebase may be added with or without water. In the latter case, the saltseparates as crystals and may be filtered off;

in-the former case, an aqueous solution or slurryof the salt settles outand may be separated by decantation and- 1 concentrated bycrystallization.

The following examples illustrate the process invention, but the latteris in no manner to be restricted to the details shown. Percentages areby weight unless otherwise stated.

Example 1 Pure phosphoric acid was used to demonstrate the improvedefiiciency of the process of this invention over that using an alcohol'alone. In Table I are listed the distribution coefficients between theorganic and aqueous phases for the extraction of phosphoric acid fromits solutions of I various initial molar concentrations into n-amylalcohol,

both with and without 10% of trmonylamme.

TABLE I Distribution of Coellieient Initial Molar Couc. p of H POl inaqueous Temp., C.

phase n-Amyl alcon-Amyl al hol 10% trinonylamlne 23 0.48 0 i i 0.45-0.30 23 O. 06 0. 31

of this In further experiments, it was shown that the improved resultswith the use of trinonylamine were not due merely to salt formation orassociation between the amine and the phosphoric acid. In Table II, col.I is the molar conc. of phosphoric acid in the organic layer using bothn-amyl alcohol and 10% trinonylamine, col. II is the amount ofphosphoric acid that could theoretically be tied up by the amine, col.III is col. I-col. II, and col. IV is the molar concentration of H POachieved by the alcohol alone. It willbe seen that col. III alwaysexceeds col. IV.

In experiments similar to those of Example 1, the following results(Table III) were obtained using 5% of trinonylamine in n-hexyl alcoholat 23 C.

TABLE III Molar Cone. of Distribution Molar Ratio,

' H3PO4 in Coeilicient, Acid/Amine in 7 aqueous phase OrgJAq. in Org.Phase 0. 36 0. 40 1. l6 0. 83 0. 23 I. 54 l. 57 O. Iii 2. ()4 2. 70 0.l5 3. 24 4. 43 0. 18 6. 25 6. 25 i 0. 256 13.

Example 3 Two 50 g. lots of a wet-process phosphoric acidcontaining-29.6% P 0 were extracted six times with either a mixture of71 g. of n-amyl alcohol and 7.4 g. of water, or a-rnixture of 7l g. .ofa 10% solution of a commercially available highly branched long chainsecondary amine of mol. wt. 35 0400 (mostly N-dodecenyl trialkylmethylamine), in n-amyl alcohol, and 7.4 g. of water. The results of theextractions are shown in Table IV.

4 Example 4 In an experiment similar to that of Example III, usingdilaurylbutyl amine in place of the highly branched secondary amine, theresults shown in Table V were obtained.

An organic acid-containing mixture was prepared from wet-processphosphoric acid by a 6-stage continuous counter-current extraction usingthe amine-alcohol mixture of Example III. The mixture was back extractedwith water in a 3-stage counter-current extraction usinga water/ feedratio of /5. 86% of the phosphoric acid was back extracted yielding anacid of the following purity.

TABLE VI Original Acid, Organic Water Extract,

Species Percent Mixture, Perceut Percent R 03 represents mixed metallicoxides, mostly iron and aluminum.

Example 6 The organic acid-containing mixture of Example 5 was strippedwith ammonia to obtain directly an equeous slurry of ammoniumphosphates. 50 g. of the organic extract were treated with 9 g. ofconcentrated aqueous arrunonia. After anaqueous slurry had settled tothe bottom, the organic phase was decanted and shown to contain onlyTABLE IV Solvent (a) 10% aminein Solvent (b) amyl alcohol alcohol alonePercent. Percent Wt. P 05 Percent Percent Wt. P 0

P 0 1120 205 H2O ExtraetNo.:'

Total P 0 g 13. 10.17 Percent P 0 extracted- 90. 6 68.8

When the acid concentration in the aqueous layer is 0.52% P 0 Theaqueous layer was evaporated to dryreduced below that corresponding toabout 20% by wt. of P 0 it is in some cases desirable to add asequestering agent to prevent the precipitation of iron and aluminumsalts. Suitable agents are polycarboxylic, hydroxy-carboxylic andamino-carboxylic acids, polyamines and polyphosphates, more specificallyoxalic acid, ethylene diamine tetraacetic acid, gluconic, lactic,citric, glycollic and salicylic acids and their sodium salts. A suitableamount of sequestering agent will usually lie between 0.1% and 5% byweight of the aqueous acid.

ness under vacuum and the solid analysed 53.2% P 0 16.6% N. Thusapproximately 95% of the phosphoric acid in the extract was obtained asa first yield of ammonium phosphates.

' Example 7 The organic extract of Example 6 was stripped with Na CO To50 g. of the organic extract was added 6.5 g. of Na CO The mixture wasgently warmed and stirred. 6.5 g. of water were added, and a clear loweraqueous phase separated. On cooling to 10 C. the phase crystallized andwas separated by filtration. After washing with alcohol it was shown tobe Na I-IPO 2H O. I he extraction removed 99.5% of the P What we claimis:

1. A process for the purification of an impure aqueous phosphoric acidsolution which comprises contacting said solution with an extractionliquid comprising 5% to 20% by weight of a water-insoluble amine and 95%to 80% by weight of analiphatic monohydric alcohol having from 4 to 12carbon atoms per molecule to form an aqueous phase and an alcohol-aminephase containing the major portion of the phosphoric acid solubilizedtherein, separating the alcohol-amine phase from the aqueous phase, andrecovering purified phosphoric acid from said alcohol-amine phase.

2. A process according to claim 1 wherein the purified phosphoric acidis recovered from the alcohol-amine phase by back extraction with water.

3. A process according to claim 1 wherein the purified phosphoric acidis recovered from the alcohol-amine phase by reaction with a base.

4. A process according to claim 1 wherein the waterinsoluble amine isselected from the group consisting of primary, secondary and tertiaryamines having a total of from to 36 carbon :atoms per molecule.

5. A process according to claim 1 wherein the phosphoric acid solutionis wet-process phosphoric acid.

References Cited by the Examiner UNITED STATES PATENTS 1,838,431 12/1931Milligan 23-165 1,857,470 5/1932 Milligan et al. 23-165 1,929,442 10/1933 Milligan 23-165 1,981,145 11/1934 Keller 23-165 2,870,207 1/ 1959Niederhauser et a1. 23-165 X 2,880,063 3/1959 Baniel et al 23-1652,885,265 5/1959 Cunningham 23-165 2,968,523 1/1961 Cunningham et a1.23-312 X 3,057,711 10/1962 Reusser et a1 23-165 X 3,076,701 2/ 1963Bersworth et al 71-43 3,082,062 3/1963 Preuss 23-312 X 3,118,730 1/ 1964Nickerson 23-165 3,129,170 4/1964 Ittlinger 210-54 OTHER REFERENCESColeman et :al.: Amine Salts as Solvent Extraction Reagents for Uraniumand Other Metals, Proceedings of International Conference of PeacefulUses of Atomic Energy, vol. 28, 1958, pages 278-288.

Irving et al.: Metal Complexes and Partition Equi- Libria, ChemicalSociety Journal (London), 1949, part 3, pages 1841-1847.

OSCAR R. VERTIZ, Primary Examiner.

O. F. CRUTCHFIELD, Assistant Examiner.

1. A PROCESS FOR THE PURIFICATION OF AN IMPURE AQUEOUS PHOSPHORIC ACID SOLUTIONK WHICH COMPRISES CONTACTING SAID SOLUTION WITH AN EXTRACTION LIQUID COMPRISING 5% TO 20% BY WEIGHT OF A WATER-INSOLUBLE AMINE AND 95% TO 80% BY WEIGHT OF AN ALIPHATIC MONOHYDRIC ALCOHOL HAVING FROM 4 TO 12 CARBON ATOMS PER MOLECULE TO FORM AN AQUEOUS PHASE AND AN ALCHOHOL-AMINE PHASE CONTAINING THE MAJOR PORTION OF THE PHOSPHORIC ACID SOLUBILIZED THEREIN, SEPARATING THE ALCOHOL-AMINE PHASE FROM THE AQUEOUS PHASE, AND RECOVERING PURIFIED PHOSPHORIC ACID FROM SAID ALCOHOL-AMINE PHASE. 